Hard disk storage devices are used in many computer system operations. In fact, most computing systems are not operational without some type of hard disk drive or similar storage device to store the most basic computing information such as the boot operation, the operating system, applications, and the like.
The basic hard disk drive model includes a storage disk or hard disk that spins at a designed rotational speed. An actuator arm is utilized to reach out over the surface of the disk. The arm carries a head assembly that has a magnetic read/write transducer or head for reading/writing information to or from a location on the disk. The transducer is attached to a slider, such as an air-bearing slider, which is supported adjacent to the data surface of the disk by a cushion of air generated by the rotating disk. The transducer can also be attached to a contact-recording type slider. In either case, the slider is connected to the actuator arm by means of a suspension. The complete head assembly, e.g., the suspension and head, is called a head gimbal assembly (HGA).
In operation, the hard disk is rotated at a set speed via a spindle motor assembly having a central drive hub. Additionally, there are tracks evenly spaced at known intervals across the disk. When a request for a read of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head reads the information from the disk. In the same manner, when a request for a write of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head writes the information to the disk.
Over the years, the disk and the head have undergone great reductions in their size. For example, the original hard disk drive had a disk diameter of 24 inches. Modern hard disk drives are much smaller and include disk diameters of less than 2.5 inches (micro drives are significantly smaller than that).
This continual reduction in size has placed steadily increasing demands on the technology used in the HGA, particularly in terms of power consumption, shock performance, and disk real estate utilization. One recent advance in technology has been the development of the Femto slider, which is roughly one-third of the size and mass of the older Pico slider, which it replaces; over the past 23 years, slider size has been reduced by a factor of five, and mass by a factor of nearly 100.
Some of the recent improvements to the head specifically have involved improving the accuracy of the sensor within the read head. One recent development in the area of the read head and sensor has been the proliferation of current perpendicular to the plane of the layers type sensors (CPP sensors). Previous sensors commonly used current in the plane of the layers instead.
In addition to the movement towards CPP sensors, it is also becoming more common to see differential sensors. Differential sensors involve two sensors, each separated by a conductive spacer (commonly made of copper). Each sensor has magnetically fixed layers and a magnetically “free” layer. The nature of the surrounding layers is such that the two free layers tend to magnetize parallel to each other. When a differential sensor reads a field from a bit, the bit is centered such that one free layer is detecting the bit's positive field and the other is detecting the negative field. The fields read by the two sensors are then summed. The advantage to differential sensors is that when a uniform field comes from the outside, any signal which is generated in both sensors is cancelled out.
New CPP sensors are not without their problems, however. In perpendicular data detection/recording, the disk typically has two layers: a recording layer and a soft under-layer. The under-layer is made of a soft material and has high permeability. Consequently, it will amplify any stray field from the outside environment. Sensors used in conjunction with shields, while proven to be insensitive to stray fields in the transverse direction, can become very sensitive to these stray fields in the longitudinal direction.